The radiation (x and gamma) sensitivity of hypoxic mammalian cells in culture can be increased close to the level of well oxygenated cells, under certain cultural conditions. Using a system of dense cultures of the HeLa cells, where hypoxia is induced by respiratory activity, we observed a potentiation of the radiation effect when plating of irradiated cells was postponed for several hours; in cells irradiated under oxygenated conditions, there was no potentiation due to delayed plating. While hypoxia seems to be essential for the radiation potentiating effect, other features in the system such as accumulation of waste products may play a role. We plan to evaluate the radiation potentiating effect of post-irradiation hypoxia as revealed in our system by varying cultural conditions, access to oxygen, size of irradiation dose, length of post-irradiation treatment both in asynchronous and later in synchronous HeLa cells. The latter can be used in conjunction with the dense culture system when mitotically synchronized cells are admixed to heavily irradiated "feeder" cells. In another approach, also aimed at selective inactivation, hypoxic synchronous HeLa cells will be irradiated in the presence of two nitroimidazole compounds, metronidazole and Ro 7-0582, which have radiosensitizing properties in absence of oxygen. In addition to their oxygen mimicking activity in radiosensitization, their synergistic effect with irradiation through toxicity to hypoxic cells will also be examined. The effect of post-irradiation hypoxia will also be studied in synchronous Chinese hamster cells. Split-dose irradiation will be conducted with both HeLa and Chinese hamster cells in order to discern how repair of sublethal radiation damage may be effected under limited oxygen supply and in the presence of hypoxic sensitizers.